This invention relates generally to methods of removing mercury from liquid hydrocarbons and is particularly concerned with methods for removing mercury from crude oil and natural gas condensates using sulfur-containing organic and/or inorganic compounds.
Natural gas and crude oils produced in certain geographic areas of the world contain mercury in sufficient quantities to make them undesirable as refinery or petrochemical plant feedstocks. For example, hydrocarbon condensates derived from natural gas produced in regions of Indonesia and Thailand often contain over 1000 parts per billion by weight (ppbw) of mercury, while crude oils from the Austral Basin region of Argentina frequently contain well over 2000 ppbw mercury. If these condensates and crudes are distilled without first removing the mercury, it will pass into distillate hydrocarbon streams, such as naphtha and gas oils, derived from these feeds and poison hydrotreating and other catalysts used to further refine these distillate streams.
In the past, adsorbents, gas stripping and chemical precipitation methods have been used to remove mercury from crudes and other hydrocarbon liquids prior to their processing in order to avoid catalyst poisoning problems. The use of fixed bed adsorbents, such as activated carbon, molecular sieves, metal oxide-based adsorbents and activated alumina, to remove the mercury is a potentially simple approach but has several disadvantages. For example, solids in the crude oil tend to plug the adsorbent bed, and the cost of the adsorbent may be excessive when mercury levels are greater than 100 to 300 ppbw. Also, large quantities of spent adsorbent are produced when treating hydrocarbon liquids having high levels of mercury, thereby making it imperative to process the spent adsorbent to remove adsorbed mercury before either recycle or disposal of the adsorbent.
Gas stripping, although simple, also has drawbacks. To be effective the stripping must be conducted at high temperature with relatively large amounts of stripping gas. Since crudes contain a substantial amount of light hydrocarbons that are stripped with the mercury, these hydrocarbons must be condensed and recovered to avoid substantial product loss. Moreover, the stripping gas must either be disposed of or recycled, both of which options require the stripped mercury to be removed from the stripping gas.
Chemical precipitation includes the use of hydrogen sulfide or sodium sulfide to convert mercury in the liquid hydrocarbons into solid mercury sulfide, which is then separated from the hydrocarbon liquids. As taught in the prior art, this method requires large volumes of aqueous sodium sulfide solutions to be mixed with the liquid hydrocarbons. The drawbacks of this requirement include the necessity to maintain large volumes of two liquid phases in an agitated state to promote contact between the aqueous sodium sulfide solution and the hydrocarbon liquids, which in turn can lead to the formation of an oil-water emulsion that is difficult to separate.
It is obvious from the above discussion that there exists a need for more effective processes to efficiently remove relatively large quantities of mercury from crude oils and other liquid hydrocarbons without the disadvantages of conventional techniques.
In accordance with the invention, it has now been found that certain sulfur-containing organic and/or inorganic compounds can be used, either directly or supported on carrier solids, to efficiently and effectively remove mercury from crude oils and other liquid hydrocarbons. In one embodiment of the process of the invention, particulate solids, such as diatomite (diatomaceous earth) and zeolites among others, on which is supported either (1) an alkali or alkaline earth metal sulfide or polysulfide, (2) an alkali metal trithiocarbonate, or (3) an organic compound containing at least one sulfur atom reactive with mercury, are mixed or agitated with the mercury-containing hydrocarbon liquids. The solids and any particulates formed during the mixing are then separated from the mixture to produce hydrocarbons of reduced mercury content.
In another embodiment of the invention, the mercury-containing liquid hydrocarbons are directly mixed or agitated with an organic compound containing at least one sulfur atom reactive with mercury, such as a dithiocarbamate, under conditions that the organic compound reacts with mercury in the hydrocarbon feed to produce mercury-containing particulates. These particulates are then removed from the mixture to produce mercury-depleted hydrocarbon liquids.
In yet another embodiment of the invention, the contaminated hydrocarbon feed is mixed with sufficient amounts of (1) an aqueous solution of an alkali metal or alkaline earth metal sulfide or polysulfide, or (2) an alkali metal trithiocarbonate such that the resultant mixture contains a volume ratio of the aqueous solution to the liquid hydrocarbon feed less than 0.003. The mercury-containing particulates formed during mixing are then separated from the mixture to produce hydrocarbons of reduced mercury concentration. Since only small volumes of aqueous solutions are utilized, it is easier to maintain the aqueous and hydrocarbon phases in intimate contact without forming detrimental emulsions and contaminating the hydrocarbons with excess sulfur.
Quite frequently the liquid hydrocarbons to be treated in the process of the invention will contain particulate matter on which a portion, sometimes over 50 weight percent, of the mercury that contaminates the liquids is adsorbed. In such cases it is normally necessary to remove the mercury-contaminated particles, usually by filtration or the use of a hydrocyclone, from the hydrocarbons before treating the remaining liquids to remove dissolved mercury.
In a preferred embodiment of the invention, a crude oil or natural gas condensate containing dissolved mercury, colloidal mercury and mercury-contaminated particulate matter is first treated to remove particulates and colloidal mercury and then mixed with a monomeric or polymeric alkyl dithiocarbamate, which reacts with the dissolved mercury to form mercury-containing particulate solids. These resultant solids are then separated from the mixture to produce a crude oil or natural gas condensate having a reduced mercury content.